Some have speculated that ulnar overgrowth in gymnasts is due to premature radial closure like in the paper Gymnast wrist: an epidemiologic survey of ulnar variance and stress changes of the radial physis in elite female gymnasts, but the case of Devon Larratt suggests the possibility of ulnar overgrowth due to dynamic loading methods.

The way the triceps is attached to the olecranon of the ulna suggests that perhaps dynamic muscle pulling of the triceps on the ulna could stimulate growth.

Ulnar variance and skeletal maturity of radius and ulna in female gymnasts

“Ulnar variance and skeletal maturity of radius and ulna in female gymnasts. Med. Sci. Sports Exerc., Vol. 31, No. 5, pp. 653-657, 1999. It is has been suggested that repetitive loading on the distal end of the radius in elite gymnasts may lead to epiphyseal changes, a premature closure (union) of the radius growth plate, and ulnar overgrowth.

It is hypothesized that ulnar overgrowth in female gymnasts is associated with advanced maturity status and early onset of epiphyseal closure of the radius, and later maturity status and later onset of epiphyseal closure of the ulnar.”<-so basically the hypothesis was that gymnastics damaged the growth plate of the radius leading to relative ulnar overgrowth.

“Posterior-anterior radiographs of 201 female gymnasts, participants of the 1987 World Championships Artistic Gymnastics, were used to measure ulnar overgrowth, to determine skeletal maturation of the hand and wrist with the Tanner-Whitehouse technique, and to determine the maturity status of the radius and ulna separately, particularly with regard to the onset of epiphyseal closure. To test the hypothesis, extreme quintiles for ulnar overgrowth were contrasted for skeletal maturation of the hand and wrist and for maturity stages of the radius and ulna as defined by the Tanner-Whitehouse criteria.”

“Female gymnasts who demonstrate ulnar overgrowth are skeletally more advanced in maturity status of the entire hand-wrist compared with gymnasts who did not show ulnar overgrowth. There were, however, no differences between gymnasts in the extreme quintiles of ulnar overgrowth in the maturation of the radius, although gymnasts with ulnar overgrowth show more advanced maturity status of the ulna.

Ulnar overgrowth is thus not apparently associated with advanced maturity of the distal radial epiphysis as defined in protocols for assessing skeletal maturity and does not apparently lead to premature epiphyseal closure of the distal radius.”<-so gymnastics did not cause premature closure of the radius so the overgrowth of the ulna must be due to something else. The hypothesis that they suggest is that the ulna grows faster.

“In gymnastics, the distal radius bears most of the force when the body mass is supported by the upper extremities. As a result, attention is commonly focused on injures of the hand and wrist, given their potential to influence the growth plates of the distal radius and ulna. It has been suggested, for example, that the repetitive loading of the distal end of the radius may lead to epiphyseal changes and premature union, and result in ulnar overgrowth. It has also been hypothesized that “repetitive cyclical injury to the radial epiphysis may cause inhibition of normal growth of the radius and/or stimulation of ulnar growth”, and that the “proposed etiology of wrist pain associated with premature fusion of the distal growth plate of the radius” is chronic overuse”

So this study only suggests that the ulna grows faster due to gymnastics via a growth plate mediated mechanism it does not suggest that mechanical loading is the cause but it does not preclude it either.